1. Field of the Invention
The present invention relates to a method for producing crystal layers of compound semiconductor fit to manufacture semiconductor lasers or light emitting diodes, and more particularly, to a method of producing an InGaAlP layer, which is a compound semiconductor crystal layer, by way of a metal organic chemical vapor deposition (referred to as MOCVD hereinbelow).
2. Description of the Prior Art
InGaAlP layers as crystal layers of compound semiconductor referred to above are important material to manufacture semiconductor lasers and light emitting diodes of a short wavelength. The MOCVD is most often used to grow the InGaAlP layer on a GaAs substrate, and there is a report on semiconductor lasers and light emitting diodes of a short wavelength manufactured according to the MOCVD, which showed favorable features and a long service life.
However, the favorable features and long life of the semiconductor laser or light emitting diode were poorly reproducible, and irregularities in characteristics of the InGaAlP layer on the GaAs substrate have been a problem. That is, a crystal layer of a high Al composition, namely, an InGaAlP layer used as a clad layer of the above-manufactured semiconductor laser or light emitting diode includes crystal defects which deteriorate the characteristics of the device.
For solving the above problem, Japanese Patent Laid-Open Publication No. 2-254715 (254715/1990) proposes a method for producing crystal layers of compound semiconductor by which the surface temperature of the GaAs substrate is raised to as high as 745.degree.-755.degree. C. thereby to grow a good InGaAlP layer without crystal defects.
FIG. 31 is a sectional view of a semiconductor laser manufactured according to the producing method proposed in the above prior art. The semiconductor laser is provided with an n-GaAs substrate 101, an n-GaAs buffer layer 102 formed on the n-GaAs substrate 101, an n-InGaAlP clad layer 103 on the n-GaAs buffer layer 102, an InGaP active layer 104 on the n-InGaAlP clad layer 103, a p-InGaAlP clad layer 105 on the InGaP active layer 104, an n-GaAs blocking layer 106 on the p-InGaAlP clad layer 105 via a p-InGap layer, and a p-GaAs contact layer 107 on the n-GaAs blocking layer 106. FIG. 32 is a diagram showing a process to form the n-GaAs buffer layer 102 and n-InGaAlP clad layer 103 by the producing method. The conventional producing method of the compound semiconductor crystal layer will now be described with reference to FIG. 32.
In the first place, an n-GaAs substrate 101 having a surface thereof purified by chemical etching is set in a reaction container (see FIG. 1) which is in turn vacuumized to 15-100 torr. Then, arsine (AsH.sub.3) is introduced into the container. The n-GaAs substrate 101 is heated and kept at 600.degree.-650.degree. C. for 30 minutes. After the substrate 101 is thus purified, TMG (trimethyl gallium) is fed into the container, whereby an n-GaAs buffer layer 102 is grown on the n-GaAs substrate 101. The growth of the n-GaAs buffer layer 102 is then stopped by shutting off the supply of TMG (step a3 in FIG. 32).
Subsequently, the temperature of the n-GaAs substrate 101 is raised to 745.degree.-755.degree. C., i.e., a growing temperature for the InGaAlP layer (step b3 in FIG. 32).
After the surface temperature of the n-GaAs substrate 101 is stabilized at the growing temperature for the InGaAlP layer, the pressure in the reaction container is decreased to 15-35 torr. Phosphine (PH.sub.3) is started to be introduced with the arsine stopped. The lapse of a time t (approximately 1 sec.) is awaited for the purpose of replacing the arsine in the reaction container. After the lapse of the time t, TMA (trimethyl aluminum), TMG and TMI (trimethyl indium) of a predetermined mixing ratio are introduced, so that an n-InGaAlP clad layer 103 is formed on the n-GaAs buffer layer 102 (step c3 in FIG. 32).
In the prior art producing method described hereinabove, the n-InGaAlP clad layer 103 is formed after the surface temperature of the n-GaAs substrate 101 is raised to 745.degree.-755.degree. C., thereby attaining the crystal layer of a high Al composition with good crystalline properties. Nevertheless, the n-InGaAlP clad layer 103 includes crystal defects represented by hillocks or the like at a high density, and moreover, As mingles in the n-InGaAlP layer 103 in the vicinity of a heterointerface with the n-GaAs buffer layer 102. As a result, a good (steep) heterointerface cannot be formed between the n-GaAs buffer layer 102 and the n-InGaAlP layer 103.
The reason why the crystal defects represented by the hillocks or the like are generated at a high density is as follows. Since the n-InGaAlP clad layer 103 is grown after the temperature of the n-GaAs substrate 101 is set at high 745.degree.-755.degree. C., P (phosphorus) is separated in the vicinity of the heterointerface of the n-GaAs buffer layer 102 and n-InGaAlP clad layer 103, whereby or by the like reason crystal defects are brought about in the early growing stage of the clad layer 103. The crystal defects affect the growth of the clad layer 103 afterwards.
The reason why As is included in the n-InGaAlP clad layer 3 at the heterointerface between the n-GaAs buffer layer 102 and n-InGaAlP clad layer 103 is that arsine in the reaction container is not sufficiently replaced with phosphine. More specifically, the n-GaAs buffer layer 102 would rapidly decompose without arsine when the surface temperature of the GaAs substrate is 745.degree.-755.degree. C. required for the formation of the good n-InGaAlP clad layer 103, and therefore it is necessary to keep the n-GaAs substrate 101 in an ambience of arsenic immediately before the clad layer 103 starts to grow. As such, after the surface temperature of the n-GaAs substrate 101 is stabilized at the growing temperature of the n-InGaAlP clad layer 103, the supply of arsine is stopped and the supply of phosphine is started, and TMA, TMG and TMI preliminarily regulated at a predetermined mixing ratio are introduced after the elapse of a short replacement time of approximately 1 sec., to start the formation of the n-InGaAlP clad layer 103. Therefore, arsine in the reaction container is not sufficiently replaced by phosphine and As is included into the n-InGaAlP clad layer 103 in the early growth stage, making it impossible to form a steep heterointerface.